According to the FBI, the new-found emails came about during an investigation into Anthony Weiner and his text messages to a 15-year-old girl. Weiner’s wife, Huma Abedin apparently used that same electronic device to send emails to Secretary of State, Hillary Clinton. The FBI is now re-addressing the issue.

The F.B.I. is investigating illicit text messages that Mr. Weiner, a former Democratic congressman from New York, sent to a 15-year-old girl in North Carolina. The bureau told Congress on Friday that it had uncovered new emails related to the Clinton case — one federal official said they numbered in the tens of thousands — potentially reigniting an issue that has weighed on the presidential campaign and offering a lifeline to Donald J. Trump less than two weeks before the election.

In a letter to Congress, the F.B.I. director, James B. Comey, said that emails had surfaced in an unrelated case, and that they “appear to be pertinent to the investigation.”

Mr. Comey said the F.B.I. was taking steps to “determine whether they contain classified information, as well as to assess their importance to our investigation.” He said he did not know how long it would take to review the emails, or whether the new information was significant.